A printing apparatus having the function of a printer, copying machine, facsimile apparatus, or the like, or a printing apparatus used as an output device for a multifunction apparatus or workstation including a computer, word processor, or the like prints an image on a printing medium such as a printing sheet or thin plastic plate (used for an OHP sheet or the like) on the basis of image information.
Such printing apparatuses are classified by the printing method used into an inkjet type, wire dot type, thermal type, thermal transfer type, electrophotographic type, and the like.
Of these printing apparatuses, a printing apparatus of an inkjet type (to be referred to as an inkjet printing apparatus hereinafter) prints by discharging ink from a printhead onto a printing medium. The inkjet printing apparatus has many advantages: the apparatus can be easily downsized, print a high-resolution image at a high speed, and print on a plain sheet without requiring any special process. In addition, the running cost of the inkjet printing apparatus is low, and the inkjet printing apparatus hardly generates noise because of non-impact printing and can print a color image by using multicolor ink.
The inkjet printing method includes several methods, and one of the methods is a bubble-jet printing method in which a heater is mounted within a nozzle, bubbles are generated in ink by heat, and the foaming energy is used to discharge ink. A printing element which generates thermal energy for discharging ink can be manufactured by a semiconductor manufacturing process. Examples of a commercially available printhead utilizing the bubble-jet technique are (1) a printhead obtained by forming a printing element on a silicon substrate as a base to prepare a printing element substrate and joining to the printing element substrate a top plate which has a groove for forming an ink channel and is made of a resin (e.g., polysulfone), glass, or the like, and (2) a high-resolution printhead obtained by directly forming a nozzle on an element substrate by photolithography so as to eliminate any joint.
Since the element substrate is made of a silicon substrate, not only a printing element is formed on an element substrate, but a driver for driving the printing element, a temperature sensor used to control the printing element in accordance with the temperature of the printhead, a driving controller for the driver, and the like may be formed on the element substrate.
The bubble-jet printing method differs from other inkjet printing methods in that a liquid which receives thermal energy is heated to generate bubbles, droplets are discharged from an orifice at the distal end of the printhead by an operating force based on generation of bubbles, and the droplets are attached to a printing medium to print information (see, e.g., Japanese Patent Publication Laid-Open No. 54-51837).
An inkjet printhead (to be referred to as a printhead hereinafter) according to the printing method using thermal energy generally comprises: a liquid discharge portion having an orifice formed to discharge liquid and a liquid channel which communicates with the orifice and is a part of a heat acting portion for causing thermal energy to act on the liquid so as to discharge droplets; a heating resistance element serving as an electrothermal transducer which is means for generating thermal energy; an upper protective layer which protects the heating resistance element from ink; and a lower layer which accumulates heat.
Such printhead requires many heating resistance elements for higher density and higher speed printing in order to exploit the features of the printhead. As the number of heating resistance elements increases, the number of electrical connections with an external wiring board increases. When heating resistance elements are arrayed at a high density, the pitch between the electrode pads of the heating resistance elements decreases, and the heating resistance elements cannot be connected by a traditional electrical connection method (wire bonding or the like).
Conventionally, this problem is solved by building driving elements for heating resistance elements in a substrate (see, e.g., U.S. Pat. No. 4,429,321). There has also conventionally been proposed a printhead which vertically discharges ink from a heat acting portion by adhering and forming an orifice plate having ink orifices onto a substrate (see, e.g., Japanese Patent Publication Laid-Open No. 59-95154).
In order to improve the removability of ink which stays on the orifice plate, and form a plurality of ink supply ports in a single substrate so as to discharge a plurality of types of inks by one substrate, such printhead is connected outside the substrate by arranging electrode pads along peripheral sides of a substrate which are parallel to short sides of the long-groove-like ink supply ports.
This configuration readily increases the wiring resistance up to the heating resistance element. If a plurality of heating resistance elements connected to the same wiring line are designed to be simultaneously drivable, the voltage drop difference greatly changes in accordance with the difference in the number of simultaneously driven heating resistance elements owing to the common resistance of the wiring line. Appropriate bubbling may not be obtained depending on image data.
For this reason, a plurality of wiring lines are so divided as to have the same resistance in manufacturing a printhead, and heaters connected to a common wiring line are time-divisionally driven so as to drive only one heating resistance element at once. This configuration suppresses the adverse effect of the common wiring line upon a change in the number of simultaneously driven heating resistance elements.
FIG. 23 is a plan view showing the structure of a conventional inkjet printhead substrate having a plurality of wiring lines.
In FIG. 23, reference numeral 1100 denotes an inkjet printhead substrate; 1104, electrode pads; and 1108, individual wiring lines.
FIG. 24 is a diagram showing the equivalent circuit of a part which forms the substrate shown in FIG. 23.
More specifically, the equivalent circuit of a part circled in FIG. 23 corresponds to the circuit shown in FIG. 24.
In FIG. 24, reference numerals 1103 denote heating resistance elements (heaters); 1107, MOS transistors serving as driving elements for driving the heating resistance elements 1103; 1104a, an electrode pad for applying a voltage for supplying energy to the heating resistance elements 1103; 1104b, a GND wiring electrode pad for supplying energy to the heating resistance elements 1103; 1104c, a voltage application power supply input pad for determining a voltage to be finally applied to the gates of the MOS transistors; and 1104d, a power supply input pad which is actually formed from a plurality of electrode pads (not shown) and drives a logic circuit. The pad 1104d includes electrode pads for GND, image data input, time division driving, and logic necessary to determine the heating resistance element driving time.
Reference numerals 1112-(1) to 1112-(n) and 1113-(1) to 1113-(n) denote individual wiring resistances generated because wiring lines are individually laid out for respectively heating resistance elements to be simultaneously driven (on the logic circuit).
Reference numeral 1109 denotes a driving element driving voltage converter serving as an element which stabilizes a voltage input from the electrode pad 1104c and if necessary, reduces the voltage; 1110, a logic circuit including a shift register (S/R), latch circuit, time division signal determination circuit, and driving time determination signal generation circuit; and 1111, a synthesizing circuit which increases a voltage of a logic control signal to the driving voltage of the MOS transistor 1107.
The MOS transistor 1107 is turned on on the basis of image data, a time division signal, a driving time determination signal, and the like which are synthesized by the logic circuit 1110 and synthesizing circuit 1111. A current then flows through the heating resistance element (heater) 1103 to generate heat by the energy, and ink is discharged by power obtained by film foaming of ink in contact with the heating resistance element 1103.
When attention is paid to a given time, only one of heating resistance elements in each portion surrounded by a dotted line in FIG. 24 is driven. In other words, when each of portions surrounded by dotted lines is regarded as a block, one of heaters belonging to each block is driven at once. This driving is called block time division driving.
The operating points of driving elements for simultaneously driven heaters will be explained with reference to FIGS. 25 and 26.
FIG. 25 shows an equivalent circuit extracted from the equivalent circuit shown in FIG. 24 for only one division part of heating resistance elements simultaneously driven by block time division driving out of a plurality of heating resistance elements.
In FIG. 25, RH represents the resistance value of one of simultaneously driven heating resistance elements; RL1, the wiring resistance value of one individual wiring line 1112-(x) (where x=1, n) shown in FIG. 24; RL2, the wiring resistance value of one individual wiring line 1113-(x) (where x=1, n) shown in FIG. 24; and RC1 and RC2, common wiring resistance values generated in an electrical wiring tape and electrical contact substrate following common wiring lines of individual wiring lines, like the electrode pads 1104a and 1104b. 
In FIG. 25, VH represents a voltage which is generated by supplying power to the heating resistance element 1103 and driving it and applied between the individual wiring line+the heating resistance element+the heater driving element (MOS transistor); IDS, a current flowing upon driving; and VDS, a voltage generated between the drain and source of the MOS transistor 1107.
Symbols “D”, “G”, and “S” around the MOS transistor 1107 represent the drain, gate, and source, respectively.
The resistance values RC1 and RC2 generated at portions other than portions on a substrate of silicon (Si) or the like exist outside the substrate, and thus the degree of freedom of design is high so that a wiring thickness can be thickened. As a result, the resistance value can be decreased.
FIG. 26 is a graph showing a current difference when a number of simultaneous driven heating resistance elements change due to fluctuation of RC1 and RC2.
A conventional heater driving element is configured to operate in the non-saturation region of a MOS transistor where the performance is high when, e.g., commonly using a power supply voltage applied to the heating resistance element. In this case, the difference in VH caused by the difference of resistance values between simultaneously driven heating resistance elements arises from only the voltage difference caused by the difference in resistance values RC1 and RC2 much smaller than the resistance value of the heating resistance element and the total current. Within this range, current variations fall within a range where ink can be stably discharged, as shown in FIG. 26.
As is apparent from FIG. 26, however, the operating point (□: for a large number of simultaneously driven heating resistance elements, ▪: a small number of simultaneously driven heating resistance elements) of the current IDS resultantly flowing through the heating resistance element changes depending on the number of simultaneously driven heating resistance elements. The current difference desirably falls within about 5% in terms of the design, and the circuit of the inkjet printhead substrate must be designed under very strict conditions.
Recently, as the inkjet printing apparatus advances in speed and image quality more and more, a printhead mounted on the apparatus and a circuit board used for the printhead must be equipped with a larger number of heating resistance elements, and the printhead must be driven at high frequencies.
In order to drive many heating resistance elements, the time division count must be increased in block time division driving. By increasing the time division count, a larger number of heating resistance elements can be driven without changing the number of wiring lines. However, the driving time assigned to each heating resistance element becomes shorter, and must be further shortened for higher-frequency driving.
In order to stably discharge ink from the printhead, energy applied to each heating resistance element must be controlled. For this purpose, a method of controlling energy applied to the heating resistance element by changing the driving time of the heating resistance element has conventionally been employed. However, even this method still requires a certain driving time, and the driving time has already reached its limit in the conventional method.
In order to increase the number of heating resistance elements without changing the driving time and drive them at the same frequency, the number of simultaneously driven heating resistance elements must be increased. Since the time division count is decreased for higher-frequency driving, the number of simultaneously driven heating resistance elements must be increased further. Hence, to increase the number of simultaneously driven heating resistance elements in the conventional wiring method, the number of individual wiring lines must be increased.
Individual wiring lines have different lengths because distances from electrode pads at the periphery of the substrate to heating resistance elements differ. To make the resistance values of individual wiring lines coincide with each other, their widths are designed such that the width is the narrowest for an individual wiring line closest to an electrode pad and becomes broader for farther individual wiring lines, as shown in FIG. 23. However, the minimum wiring width is limited by the manufacture, and a thicker wiring line is required as the number of wiring lines increases. In practice, when the number of simultaneously driven heating resistance elements is doubled, the wiring width increases three or four times, resulting in an abrupt increase in substrate size.
In the future, the number of heating resistance elements of the printhead will increase, and higher printing speeds will be required. Along with this, the number of simultaneously driven heating resistance elements inevitably increases. Thus, VH voltage fluctuation depending on the difference in the number of simultaneously driven heating resistance elements caused by the common wiring lines RC1 and RC2 as shown in FIG. 25 becomes large. This adversely affects the stability of ink discharge and the durability of the printhead.
Another problem will be discussed.
FIG. 27 is a block diagram showing a representative example of the configuration of an element substrate for a conventional inkjet printhead (see U.S. Pat. No. 6,116,714).
As shown in FIG. 27, an element substrate 900 comprises a plurality of heaters (printing elements) 901 which are parallel-arrayed and supply thermal energy for discharge to ink, power transistors (drivers) 902 which drive the heaters 901, a shift register 904 which receives externally serially input image data and serial clocks synchronized with the image data, and receives image data for each line, a latch circuit 903 which latches image data of one line output from the shift register 904 in synchronism with a latch clock and parallel-transfers the image data to the power transistors 902, a plurality of AND gates 915 which are respectively arranged in correspondence with the power transistors 902 and supply output signals from the latch circuit 903 to the power transistors 902 in accordance with an external enable signal, and input terminals 905 to 912 which externally receive image data, various signals, and the like. Of these input terminals, the terminal 910 is a printing element driving GND terminal, and the terminal 911 is a printing element driving power supply terminal.
The element substrate 900 further comprises a sensor monitor 914 such as a temperature sensor for measuring the temperature of the element substrate 900, or a resistance monitor for measuring the resistance value of each heater 901. A printhead in which a driver, a temperature sensor, a driving controller, and the like are integrated in an element substrate has already been commercially available, and contributes to improvement of the printhead reliability and downsizing of the apparatus.
In this configuration, image data input as serial signals are converted into parallel signals by the shift register 904, output to the latch circuit 903, and latched by it in synchronism with a latch clock. In this state, driving pulse signals for the heaters 901 (enable signals for the AND gates 915) are input via an input terminal, and the power transistors 902 are turned on in accordance with the image data. A current then flows through corresponding heaters 901, and ink in the liquid channels (nozzles) is heated and discharged as droplets from orifices at the distal ends of the nozzles.
FIG. 28 is a view showing in detail a part associated with variations in parasitic resistance on the element substrate for the inkjet printhead shown in FIG. 27.
A parasitic resistance (or constant voltage) component 916 which leads to a loss in supplying energy to the printing element upon application of a constant power supply voltage from the printing apparatus main body exists in the power transistor 902 (which is a bipolar transistor in this case, but may be a MOS transistor) shown in FIGS. 27 and 28, and a common power supply wiring line and GND wiring line for driving a plurality of printing elements. Further, in areas 2801 and 2802 encircled by broken lines as shown in FIG. 28, a voltage generated by the parasitic resistance 916 changes depending on the number of simultaneously driven heaters 901, and as a result, energy applied to the heater 901 varies.
The area 2801 contains a parasitic resistance component 2801a present in a power supply wiring line of the inkjet printing apparatus, a parasitic resistance component 2801b present in a power supply wiring line of the inkjet printhead, and a parasitic resistance component 2801c in a common power supply wiring line. Likewise, the area 2802 contains a parasitic resistance component 2802a present in a GND wiring line of the inkjet printing apparatus, a parasitic resistance component 2802b present in a GND wiring line of the inkjet printhead, and a parasitic resistance component 2802c in a common GND wiring line.
In practice, as shown in FIG. 28, the heaters 901 serving as printing elements inevitably vary in absolute resistance value by ±20% to 30% in mass production owing to the difference in film thickness and its distribution in the substrate manufacturing process.
From this, a power transistor has been used as a driver for driving the printing element of an available inkjet printhead in order to mainly reduce the resistance. The power transistor 902 functions as a constant power supply having an opposite bias to a constant element driving power supply, or an ON resistance. Since a current flowing through the printing element 901 changes depending on variations in the resistance of the printing element, energy (power consumption) applied to the printing element during a predetermined time greatly changes depending on the resistance value of the printing element in the manufacture.
The energy change has conventionally been coped with by changing by the resistance of the printing element a pulse width applied to drive the printing element. With this measure, power consumption of the printing element is made constant so as to stably discharge ink by driving the inkjet printhead and achieve a long service life of the printhead.
In recent years, the number of necessary printing elements greatly rises for higher printing speed. At the same time, it becomes more necessary than a conventional printing apparatus to uniform energy applied to the printing element for higher printing resolution. As described above, as the difference in the number of simultaneously driven printing elements becomes larger, energy applied to the printing elements more greatly varies, and the service life of the printhead becomes shorter. This generates a fault such as degradation of the printing quality owing to energy variations.
As a recent technique, the driver part is so controlled as to supply a constant current to each heater in a configuration having an effect of making energy constant, as shown in FIG. 29. This configuration can solve the above-described problem because a constant current always flows through each heater and energy, i.e., (resistance value of heater)×(square of constant current value) is supplied regardless of the number of simultaneously driven printing elements unless the resistance value varies during use. A configuration which keeps a current flowing through the heater constant has also been proposed (see, e.g., U.S. Pat. No. 6,523,922).
Among the printhead substrates, the resistance of the printing element (heating resistance element) which is the largest among resistance components varies by about 20% to 30% owing to manufacturing variations, as described above. Note that the same reference numbers are added to the same constituent elements or matters as those described in FIGS. 27 and 28, and the description is omitted. Since the power supply voltage of the printing apparatus main body in a conventional mechanism is constant, energy applied to the printing element is made constant by adjusting a pulse width applied to the printing element upon variations in the resistance of the printing element, as also described above.
However, when a constant current is commonly supplied to the heaters of a plurality of substrates in order to eliminate variations in energy caused by the difference in the number of simultaneously driven printing elements, like the prior art, the power loss on the inkjet printhead substrate by variations in the resistance of the printing element greatly changes.
FIG. 30 is a table showing variations in power loss when the printing element is driven at a constant current.
The example shown in FIG. 30 assumes variations in voltage generated at both ends of the heater and manufacturing variations in heater (in this case ±20%) when the resistance value of the printing element is about 100Ω and a 150-mA current is supplied as a constant current. FIG. 30 shows the ratio of energy consumed by constituent components other than the printing element when the printing element has a maximum resistance (120Ω), 1 V is necessary to control the driver voltage for a voltage (18 V) between both ends of the printing element, and a voltage (19 V) higher by 1 V is applied on the printing apparatus side in order to control a constant current. The power consumption of the printing element upon supply of a constant current changes (1.8 to 2.7 W) depending on variations (80 to 120Ω) in the resistance value of the printing element. Upon variations, application power is adjusted by changing the pulse width applied to the printing element in actual printing.
FIG. 30 also shows pulse widths necessary when energy is made constant.
In FIG. 30, as indicated in a dotted area 3001, when the resistance value of the printing element is 80Ω, about 58% of power applied to the printing element is mainly consumed (power loss) by a control part (driver part in the inkjet printhead substrate) for supplying a constant current. In order to make energy applied to the printing element constant even though the resistance value changes, the application pulse width is adjusted to 1.25 μs for a printing element resistance of 80Ω and 0.83 μs for a printing element resistance of 120Ω. As understood from a comparison between values in dotted areas 3002 and 3003, the ratio of these application pulse widths is about 1.5 times, and the difference in loss energy is different by about 10 times between the printing element resistances of 80Ω and 120Ω.
Particularly, when the resistance value of the printing element is 80Ω, about 58% of energy applied to the printing element is lost. On the other hand, when the resistance value of the printing element is 120Ω, the lost is about 6%. Thus, heat generated in the substrate also varies depending on the resistance value of the printing element.
If all the power is consumed within the inkjet printhead substrate, the substrate temperature goes up. This influences the ink discharge amount.
FIG. 31 is a graph showing the relationship between the printing time and the substrate temperature when a constant current is supplied to the inkjet printhead substrate.
As is apparent from FIG. 31, the degree of rise of the substrate temperature changes upon variations in the resistance of the printing element.
FIG. 32 is a graph showing the relationship between the ink temperature and the ink discharge amount.
As is apparent from FIG. 32, as the ink temperature changes, the ink discharge amount also changes. Since the ink temperature is influenced by the substrate temperature, the rise of the substrate temperature influences the ink discharge characteristic.
Hence, the fact that variations by about 20% to 30% in the resistance value of the printing element in manufacturing the printhead cannot be avoided means that it is very difficult to provide an inkjet printhead having uniform ink discharge performance.
As described above, when the method of driving the printing element at a constant current in order to eliminate the difference caused by a change in the number of simultaneously driven printing elements is introduced, energy is wastefully consumed owing to variations in the resistance value of the printing element in the printhead manufacturing process. Moreover, in actual printing, the temperature variation characteristic of the substrate changes, and the printing performance of the printhead greatly varies upon a change in ink viscosity or the like depending on the ink temperature.